The Environmental Protection Agency ruled yesterday that new power plants are not required to install technology to reduce carbon dioxide emissions, rejecting an argument from environmental groups.

    The ruling, in a memorandum signed by EPA Administrator Stephen L. Johnson, turns on a seemingly arcane regulatory question that could govern the future of new fossil fuel-burning buildings and power plants under the Clean Air Act.

    During the Bush administration, the EPA has rejected the idea that greenhouse gases should be regulated like soot, smog precursors and other kinds of air pollution, despite an April 2007 Supreme Court ruling that said carbon dioxide fit the definition of a pollutant that could be regulated under the Clean Air Act.

    The case at issue yesterday began in 2007 when the EPA issued a permit to a new coal-fired power plant in Bonanza, Utah. The Sierra Club, an environmental group, filed a legal challenge, saying that the permit should have required the plant to control its output of carbon dioxide.

    In a case before the EPA's Environmental Appeals Board, the Sierra Club cited a rule that required plants to use the best available technology to control all "regulated" pollutants, as well as the April 2007 Supreme Court ruling.

    The case revolved around the question of whether carbon dioxide was required to be controlled or simply monitored.

    The appeals board, a kind of appeals court for EPA rules, found on Nov. 13 that the rule was unclear. Johnson's memo yesterday sought to make it plain. Major industrial corporations have been pressing the Bush administration to issue a ruling in the case.

    "That is our established interpretation," Robert Meyers, the head of the EPA office of air and radiation, said in an interview. "We've been applying it that way for 30 years."

    It was unclear yesterday what the ruling's real-world impact will be. The EPA says that about 50 plants - either new or significantly remodeled - must obtain a permit under this provision every year. But Meyers said he does not know if any are positioned to receive final approval before President-elect Barack Obama takes office on Jan. 20.

    The Obama administration is likely to review the case, and Democratic officials close to the president-elect's team say that the Supreme Court ruling and the EPA's power to regulate carbon dioxide can serve as powerful levers to bring corporations and other parties to a bargaining table about broad framework for controlling greenhouse gases.